1. Field of the Invention
The present invention relates to a printing apparatus such as thermal printer.
2. Description of the Prior Art
In thermal printer there is widely used such kind of heat sensitive paper coated with a substance which is able to develop color under the action of heat.
However, use of such heat sensitive paper in thermal printing involves the following disadvantages:
A. Only one sheet of record is obtainable. PA1 B. A careful preservation of recording paper is required. The recording paper is apt to easily develop color under a high temperature condition or by contact with chemicals such as alcohol, thinner and the like. PA1 C. Printed characters or recorded images become faded when exposed to direct sunlight or contacted with plasticizer such as that of vinyl chloride for a long time. PA1 D. A close adhesion of the second sheet to the first sheet should be assured. Otherwise, a thin print is obtained on the second sheet. PA1 E. Because of the close adhesion, it is not easy to separate the two sheets.
In order to solve the problem of A, that is, the disadvantage that only one sheet of record is obtainable, a combination of two or more sheets of heat sensitive paper has been already proposed. An example of such combination is shown in FIG. 1. As seen there, two sheets of heat sensitive paper SP1 and SP2 are adhered together. By separating the two sheets SP1 and SP2 after printing there are obtained two sheets of record. However, this solution concerns the problem A only. Other problems of B regarding the preservation of record and C regarding fading remain unsolved. Rather, for such combination of sheets of heat sensitive paper, there arise the following new problems:
Since these two problems are contradictory ones, it is very difficult to find an effective solution which can solve the above two problems at the same time. A known system proposed to solve the problems is shown in FIG. 2. This system is featured by the fact that the first sheet SP is a sheet of heat sensitive paper the backside surface of which is coated with a layer of heat fusible ink T and the second sheet PP is of a mere common paper. This system has many advantages as compared with the first mentioned one.
First of all it is no longer necessary to stick the sheets together. Secondly, the pressure required to effect printing on the two sheets by pressing the thermal head against the platen through the sheets need not be so large. A pressure nearly equal to that required to effect printing on one single sheet is sufficient to make clear prints on the two sheets. Thirdly, it has no problem in respect to heat transmission since printing on the second sheet is effected by fusing and transferring the ink coated on the backside of the first sheet.
FIG. 3 illustrates the manner of printing in accordance with the method. Designated by 1 is a thermal head. 2 is a sheet of heat sensitive paper which constitutes the first sheet in this system. When dots 1' on the thermal head 1 generate heat, a chemical reaction takes place in a heat sensitive layer 2' coated on the upper surface of the sheet 2 so that the layer develops color. At the same time, heat is transmitted to the backside of the sheet to fuse the ink 2". 3 is a simple common paper which constitutes the second sheet of paper in this system. The pressure applied by the thermal head 1 keeps the two sheets 2 and 3 in a state of close contact with each other and therefore the fused ink 2" is sandwiched in between the two sheets. When the heat generation of the thermal head ends, the ink hardens. The two sheets come out with their printed portions being adhered together by the ink. Since unprinted portions of the two sheets remain separated from each other, the separation of the two sheets after printing can be done very easily. Usually, the backside surface of the first sheet 2 is pretreated, for example, by coating it with wax and the like prior to coating with the layer of ink 2" so as to assure an easy separation of the ink layer from the first sheet after printing. Therefore, at the time of the two sheets being separated after printing, the fused portions of the ink layer are easily transferred onto the second sheet 3.
As will be clearly understood from the foregoing, with the improved system of combination of printing papers, the above mentioned problems D and E involved in the first mentioned type of combination of printing papers are completely solved. Moreover, as far as the second sheet is concerned, other problems of fading and preservation as previously mentioned also can be solved. This is because the composition of the ink used in this system is almost the same as that of inks widely used in carbon paper and the like which has no problem of fading. Even when the print is left under a condition of high temperature, no gradation occurs. On the contrary, the record on the sheet becomes much more stable since the ink is refused and sinks in the paper thoroughly under a condition of high temperature.
FIGS. 4 and 5 show two different forms of the above described double sheet type of recording paper system. In FIG. 4 showing a roll of recording paper, 2 is a heat sensitive paper as the first sheet and 3 is a common paper as the second sheet. These two papers 2 and 3 are rolled round a common shaft and the length of web of the first paper 2 is longer than that of the second one 3, by 2 .pi.x (thickness of the paper) per round. This difference in length between the first and second papers 2 and 3 in one roll causes trouble. Since, as previously described with reference to FIG. 3, the first and second sheets 2 and 3 come out, after printing, from the area of the thermal head 1 with the printed portions adhered together by ink and with the same length, a slack is necessarily formed in the first sheet 2. When the slack becomes large, difficulty such as paper jamming is caused thereby.
FIG. 5 shows another form of the duplicate recording paper comprising the first sheet of heat sensitive paper 2 and the second sheet of common paper 3 which have a number of sprocket perforations along both sides and which are folded regularly. In this case, the length of the first sheet 2 may be the same as that of the second one. However, as well known to those skilled in the art, the size of a sheet of paper suffers a change by temperature and moisture. In the combination shown in FIG. 4 or 5, a paper coated with particular substances on both sides is used as the first sheet 2 whereas the second sheet 3 is of a bare common paper. Therefore, the difference in expansion or shrinkage between the two sheets is not small. The difference in length produced by it between the two sheets reaches often the order of 1 mm per 30 cm in the worst case it may reach the order of 5 mm per 30 cm. This is the same for the roll type one shown in FIG. 4.
Such difference in size between the two sheets brings forth a very serious problem, in particular, for the fold type recording paper system shown in FIG. 5. If there is produced a difference in length of 1 mm per 30 cm, then, after printing by 1.2 m, the difference will increase up to 5 mm. This makes the sprocket perforations on the first sheet 2 and those on the second one 3 get out of register which may result in breaking of the paper.